Vocabulary

What counts as one's vocabulary? The words they use and choose to use in their writing? Or does it--and should it--include words which they are familiar with, and therefore might come across, in someone else's writing?

I enjoy many word games and need to have a pretty wide vocabulary for these games. Sometimes I run across a word which I have never seen before, and therefore isn't included in my vocabulary, or word knowledge. I try to make a mental note of it, but if I have nothing to relate it to, it doesn't stick.

When I was in school many years ago now, I was forced to read and learn Latin, and one of the advantages of that was to learn derivation of words, so it's easier to interpret words if they are from a Romance language. Does that mean I could technically have an even greater vocabulary insofar as many words might share a common root?

Tell me what you think, and how you define your language ability.